feat: support object-level descriptions

By saving them as a pydantic model docstring.

Fixes https://github.com/HideyoshiNakazone/jambo/issues/77

tests/test_schema_converter.py

@@ -274,6 +274,7 @@ class TestSchemaConverter(TestCase):
         }
 
         model = self.converter.build_with_cache(schema)
+        self.assertEqual(model.__doc__, "A person")
 
         obj = model(address={"street": "123 Main St", "city": "Springfield"})